Motor learning in apraxia: voxel-based lesion symptom mapping (VLSM)

Although apraxia is commonly understood as a dysfunction of learned movements, there have been only few investigations targeting motor learning in patients suffering from apraxia. Using the serial reaction time task we were able to show that apraxic patients are capable of incidentally learning new motor sequences. Interestingly, patients with apraxia are impaired in consciously recalling the incidentally learned motor sequence knowledge. Dysfunction of consciously recalling the motor sequence information is associated with lesions in the dorsal premotor cortex (and the underlying white matter, see part C of the Figure (blue areas)). This part of the brain was more often affected by the stroke in patients with apraxia compared to patients without apraxia (see part C of the Figure (orange areas)).

Gesture processing deficits constitute a key symptom of apraxia, a disorder of motor cognition frequently observed after left-hemispheric stroke. The clinical relevance of apraxia stands in stark contrast to the paucity of therapeutic options. Transcranial direct current stimulation (tDCS) is a promising tool for modulating disturbed network function after stroke. We investigated the effect of parietal tDCS on gesture processing in healthy human subjects.
Neuropsychological and imaging studies suggest that the imitation and matching of hand gestures involves the left inferior parietal lobe (IPL). Using neuronavigation based on cytoarchitectonically defined anatomical probability maps, tDCS was applied over left IPL areas PF, PFm or PG in healthy participants (n=26). Before and after tDCS subjects performed a gesture matching task and a person discrimination task for control. Changes in error rates and reaction times were analyzed for the effects of anodal and cathodal tDCS (compared to sham tDCS).
Matching of hand gestures was specifically facilitated by anodal tDCS applied over cytoarchitectonically defined IPL-area PFm, while tDCS over IPL-areas PF and PG did not elucidate significant effects. Taking into account tDCS electrode size and the central position of area PFm within IPL, it can be assumed that the observed effect is rather due to a combined stimulation of the supramarginal and angular gyrus than an isolated PFm-stimulation. Our data confirm the pivotal role of left IPL in gesture processing. Furthermore, anatomically guided tDCS of left IPL may constitute a promising approach to neurorehabilitation of apraxic patients with gesture processing deficits.

Anatomical location of the individual tDCS sites
The individual location of the tDCS electrode’s centre normalized to MNI standard coordinates and projected onto the surface rendering of the MNI standard template brain are shown for all 26 subjects. The colour coding indicates the cyto-architectonically defined area to which the individual stimulation sites could be assigned using the maximum probability maps (MPMs) provided by the Anatomy toolbox in SPM (blue = PF, red = PFm, and yellow = PG). In addition, for three selected central stimulation sites within the areas PF, PFm and PG, a square is depicted that represents the extent of the stimulation electrode used in the current study projected on the surface rendering of the standard brain with the help of the software package Brainsight (Rogue Research Inc., Montreal Quebec, Canada).
Note that only anodal tDCS of area PFm (red dots and square) facilitated gesture processing.